This invention relates to a process for the conversion of carbonaceous materials, such as coal, to liquid products. More particularly, this invention relates to a process for the conversion of coal to a liquid product by solvent extraction of the coal using a heavy hydrocarbonaceous liquid containing heptane-insoluble material and a hydrocarbonaceous recycle stream, and recovery of the mixture of the solvent and liquified coal as the product of the process.
Resources of solid carbonaceous substances such as coal, lignite, oil shale, etc. represent a valuable source of raw materials for the production of liquid hydrocarbon products commonly obtained from petroleum. The relative abundance of sources of solid carbonaceous materials with respect to those of petroleum, makes the use of these solids to supplement and replace petroleum as energy sources economically desirable.
Several processes for converting coal to valuable liquid products are known to the art. Recently, high pressure hydrogenation and solvent extraction techniques have been developed, the latter of which is related to the process of this invention. In the processes of solvent extraction known to the prior art, crushed, finely-divided particulate coal, or other carbonaceous material, is placed in contact with a liquid solvent which dissolves a part of the solid, usually in the presence of hydrogen gas. Following the contact, the liquid solvent and the liquified part of the solid are separated from the remaining solid material by filtration, centrifuging or a similar operation. In the other processes known to the prior art, the previously solid material is separated from the solvent, typically by fractional distillation, and is further processed by conventional hydrocarbon processing techniques such as coking, cracking, hydrogenation, etc., to convert the solvent extracted material into more useful products.
A pertinent prior art reference is U.S. Pat. No. 3,705,092 which teaches a process for the extraction of coal with heavy hydrocarbonaceous liquids while simultaneously improving the quality of the hydrocarbonaceous liquids. The present invention recognizes the fact that the prior art process is improved by recycling at least a portion of the reaction zone effluent.
Another example of the typical prior art in the area of coal hydroconversion is U.S. Pat. No. 4,077,867.
One of the problems encountered in the solvent extraction method of liquefying solid carbonaceous substances is the non-selective nature of the solvation which takes place. The process is intended to extract the most valuable, hydrogen-rich fraction of the solid. But solvents which are effective in extracting this hydrogen-rich fraction also liquify an undesirable fraction containing asphaltenes. Asphaltenes are undistillable compounds of carbon of high molecular weight, and contain less than about 7% hydrogen by weight. Asphaltenes are also insoluble in normal heptane. They are present not only in the products from solvent extraction of carbonaceous materials such as coal, but also in petroleum crude oil and fractions thereof such as topped or reduced crude oils, heavy cycle stocks visbreaker liquid effluent and the bottoms from atmospheric crude towers. The asphaltenic fractions of all these liquid hydrocarbons are of little intrinsic value and interfere with the processing of the more valuable heavy oil fractions with which they are mixed. Thus, the reduction of the asphaltene content in any processable hydrocarbon liquid is a desirable improvement thereof. Further conversion of asphaltenes to distillable hydrocarbons is possible by recycling at least a portion of the reaction zone effluent.